1. Field of the Invention
This invention relates generally to machines and procedures for printing text or graphics in color on printing media such as paper, transparency stock, or other glossy media; and more particularly to a system and method for determining presence and location, on a printing medium, of ink that is of a color invisible to an optical sensor.
Throughout this document, in referring to ink that is invisible to a sensor we implicitly refer to observations of ink coated onto some particular printing medium under some particular illumination. For present purposes--namely, enhancement of calibration-pattern detection for determining positional errors of marking implements such as printheads--the printing medium is ordinarily white paper and the illumination is bright green light from a common and industrially popular light-emitting diode that emits with a peak at 560 nm.
For other purposes, or for other combinations of print medium and illumination--and in particular for other combinations of inks--the specific preferred numerical ranges mentioned in this document will likely require modification even though the fundamental implementation of our invention remains valid.
Furthermore, in referring to color that is invisible to a sensor we mean color that does not itself provide adequate contrast--relative to the printing-medium background without the color--for adequately reliable detection by the sensor. As used here, "contrast" is evaluated within the effective waveband established by the illumination, sensor sensitivity and printing-medium background. As will be seen, our invention artificially elevates such contrast.
The invention is useful particularly but not exclusively in scanning thermal-inkjet printers that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array.
2. Related Art
Automatic sensing of printed image details in a modern computer-controlled desktop printer or draftingroom plotter may be desired for various reasons, such as determining whether a particular printhead or nozzle is laying down ink:
at a nominal position for that head or nozzle (and, if not, then where); or PA1 in the nominal inking density or flow volume; or PA1 at all. PA1 registration of image components (most commonly in a multipass plotter) to each other--or to a preprinted registration grid. All these automatic-sensing applications have become increasingly important commercially with the modern trends toward increased overall automaticity, finer image resolution, and registration tolerances.
The related patent documents enumerated earlier describe systems and methods for the first of these purposes--i.e., using a sensor system to check the inking position of a printing device.
In addition to these three closely related purposes, automatic sensing is used for:
In some cases, however, problems may arise when the functions of equipment modules (illuminators and sensors) initially designed into a printer for one use, such as for example merely sensing registration marks printed in black ink, may be expanded to handle some of the other tasks as well. As mentioned above, in a four-color system such other tasks may include, for example, checking ink density for several marking implements that print in various colors respectively.
Systems which evolve in this way may not be well adapted to locating indicia printed in some of the system colors. Spectral emission and sensitivity for light sources and sensors originally selected for economy and efficiency in sensing black indicia may turn out to be blind to some ink colors.
Furthermore, even in a new system, designing around spectral mismatches may become expensive or awkward, since otherwise-ideal narrowband sources or sensors may be inefficient for some spectral regions. Some green or red light-emitting diodes, for example, are popular for their low cost and reliable operation--but magenta ink on white paper may be invisible under red light, and yellow on white paper may be nearly invisible under green light.
Heretofore it has been possible to avoid these mismatches only by resorting to sensors or sources (or both) that are relatively expensive or have other operating drawbacks; or by providing an optical filter and appropriate corresponding source, at additional cost, to create the necessary spectral distinctions.
Thus there remains room for useful and important refinement, in making all colors in a multicolor printing system detectable by commonly used and otherwise desirable sensor/source combinations.